1. Field of the Invention
The present invention relates to a glass window antenna for a motor vehicle, wherein antenna conductors are arranged on a surface of a window glass used as an insulator.
2. Description of the Prior Art
In an antenna system of a motor vehicle, a diversity reception using a plurality of antennas is employed for reducing changes in reception gain according to traveling direction of the vehicle. An FM broadcast diversity reception, for example, a pole antenna attached on the body of the motor vehicle and an antenna comprising an antenna conductor attached on a rear glass window are used in a prior art. A space diversity reception is so performed that the highest level one in reception signals is selected (cf. Japanese laid open patent application No. 140301/1985).
It is known to perform diversity reception with two or more antenna conductors and feed terminals on a rear window surface of the motor vehicle (Japanese utility model registration application laid open Nos. 138408/1988 and 29307/1988 and a patent application laid open No. 269625/1988).
As to the pole antenna, tuning adjustment is possible only with regard to its length. It is less flexible in arrangements of conductor than that of a glass window antenna. It is therefore difficult to tune the conductor for providing complementary directivity for diversity reception system. An element of the pole antenna is arranged vertically so that it shows a low reception sensibility to receive horizontally polarized wave.
It is possible to tune directivity of respective antennas in diversity reception system in which spaced conductors arranged on a surface of a glass window. Spaces on the window, however, are little for the antenna conductors so that they show low average reception sensibility. Preamplifiers with fixed gains must be inserted immediately after feed terminals of the antenna conductors. These fixed gain amplifiers often saturate at strong radio wave field so that reception radio voice is remarkably degraded.